Abstract
In a boundary layer formed on the desalting surface of an ion exchange membrane, natural or forced convection is produced. The ionic transport in the boundary layer is, accordingly, reasonable to be expressed by the extended Nernst-Planck equation taking account of the convection term. Limiting current density is not only the phenomenon of a membrane but also that of an apparatus. The limiting current density of the membrane is measured using the apparatus in which the flow pass length in a desalting cell is small or the distance between the membranes is large. Limiting current density of an electrodialyzer is obtained under the assumption that the distribution of solution velocities in each desalting cells are expressed by the normal distribution, and that the limiting current density condition is realized on a cation exchange membrane at the outlet of a desalting cell in which the solution velocity is the least.
Water dissociation arise in a water dissociation layer formed at the interface between a membrane and a boundary layer at above the limiting current density. The water dissociation arisen in the water dissociation layer is extremely accelerated comparing the phenomenon in a solution. The water dissociation arisen on an anion exchange membrane is generally more accelerated than that on a cation exchange membrane. This is because the second Wien effect of quaternary ammonium groups in the anion exchange membrane is larger than that of sulfonic acid groups in the cation exchange membrane. On a cation exchange membrane placed in a magnesium chloride solution, however, violent water dissociation occur. This is brought about by catalytic chemical reaction of OH groups in magnesium hydroxides precipitated on the desalting surface of the cation exchange membrane.
